Priority is claimed from United Kingdom application 8411095, filed May 1, 1984.
This invention relates to bottle grade polyester resins--in particular bottle grade polyethylene terephthalate (PET) resins--reinforced with glass fibers and to containers made of such resins.
It is known that many kinds of high molecular weight polyester resins, in particular PET resins, are widely used for molding plastic articles. Resins of this type and their production are described in U.S. Pat. No. 4,123,415.
By making use of these resins it is possible to produce many kinds of shaped articles such as sheets, plates, and other parts. Bottles and containers can be made by injection or compression moulding, by extrusion blow molding, or by stretch-blowing molding. In injection or compression molding, the resin is injected or compressed into the cavity of a mold from which bottles or containers are removed after the resin solidifies in a crystallized form.
In injection stretch-blowing the resin to be used must have suitable features and is of a type which is known in the art with the name "bottle grade". In stretch-blowing molding, the first step is the production of a preform. Thereafter, the preform is transformed into a finished container by preheating it to about 90.degree. to 100.degree. C. and then mechanically stretching and blowing the preform in a mold having the configuration of the final container. In the containers obtained with the stretch-blowing molding procedure, the polymer chains of the resin are oriented in two directions at 90.degree. to each other, along the longitudinal axis and along the circumference of the container, and are thus referred to as being biaxially oriented or bioriented. Biorientation of the polymer chains greatly improves the strength and mechanical properties of the container, and also reduces the gas permeability of the container. For this reason, bioriented PET containers are used to package many marketable goods, particularly foods and beverages.
Furthermore, in the containers obtained by stretch-blowing, the resin is in amorphous form which is important for the best clarity and transparency of the containers. However, such bioriented PET bottles will shrink or distort their shape when filled with hot materials, or when filled and sealed with a pressurized product and heat-treated to pasteurize the product as in the case of beer or orange juice. When the pressurized materials are pasteurized, the increase in temperature will produce an increase in the internal pressure and a decrease of the wall strength of the container. This combined effect leads to distortion or even to the failure of the container.
It is known that the strength and thermal stability of polyester resins may be improved by intimately blending such resins with reinforcing fibers, such as glass or quartz fibers. See for example, the PET-fiber glass compositions disclosed in U.S. Pat. No. 3,814,725, containing 20 to 60% by weight of fiber glass.
The increased strength of such PET-fiber glass compositions, however, prevents them from being stretched and blown on machines such as described in U.S. Pat. No. 4,105,391 where PET preforms are generally heated to about 90.degree. to 100.degree. C., and subjected to the action of a stretch-blow rod which exerts an axial mechanical pressure on the parison and injects pressurized air to stretch and blow the parison into the shape of a bottle mold.
Lowering the fiber glass content of such compositions could decrease their strength to a point where they may be stretched and blown on such machines. However, the lower the percentage content of fibers in the resin the worse are the mechanical features of such compositions.
This is confirmed by lines 44-66, col. 1 of U.S. Pat. No. 4,123,415 wherein it is stated that it is known that when glass fibers having a standard diameter of 0.00051 to 0.00055 inches are used in various thermoplastic polyester compositions where the glass fiber concentration is less than 20%, the deflection temperature under load is markedly reduced. This reduction in the heat deflection temperature under load is undesirable when the thermoplastic polyester is to be used in applications involving exposure to rather high temperature. This means that in such reinforced resins the concentration of glass fibers, with the fibers having the above referred diameter, can be reduced to such a low amount that the same could perhaps be used for producing, at a temperature of about 100.degree. C., containers with the stretch-blowing process. Such containers would, however, be surely unsuitable for use where hot-filling or heat sterilization before filling are required, especially when the container has to be filled with a substance which has to be pasteurized or the like, as explained in the foregoing. Indeed such containers would be surely distorted or they could even fail under the combined effect of the increase of temperature and of internal pressure.
This is confirmed by the fact that all available commercial data suggests that at least 10% of fiber glass has to be employed to obtain any significant improvement in the mechanical properties and thermal stability in polyester resin-fiber glass compositions.
In order to improve retention of heat deflection properties of reinforced thermoplastic resins, U.S. Pat. No. 4,123,415 has proposed to reinforce said resins with glass fibers having a diameter lower than 0.0005 inches, said glass fibers being present in an amount (from 1% to about 60% by weight of the combined weight) at least sufficient to improve the deflection by heat of said fibers. In other words, said patent claims glass fibers reinforced resins in which the heat deflection temperature is reduced to a much lesser extent in comparison with previously known reinforced resins. Indeed, as seen from the examples forming part of the U.S. Pat. No. 4,123,415, the reinforced resins disclosed therein have heat distortion temperatures which are much higher than those obtainable according to the prior art.
U.S. Pat. No. 4,123,415 reports data indicating that improved PET properties are achieved using fiber glass concentrations as low as 7% by weight, provided the average diameter of the fiber glass is maintained below 0.0005 inches. That patent alleges that as little as 1% of fiber glass having said diameter may be used to improve the aforesaid properties, but extrapolation of the actual test data of examples 1 to 4 of the patent does not suggest any beneficial effect at such low concentrations of fiber glass.
Since the object aimed at by said patent is to obtained reinforced resins having improved resistance to deflection by heat, it is obvious that glass fibers can advantageously be used bundled into yarns or ropes or woven into mats. Indeed, in col. 4, lines 12-16 of the U.S. Pat. No. 4,123,415 there is specified that "the length of the glass filaments and whether or not they are bundled into fibers and the fibers bundled in turn into yarns, ropes or rovings, or woven into mats, and the like, are also not critical to the invention".
It is also obvious that the reinforced resins according to the U.S. Pat. No. 4,123,415 are not suitable for stretch-blow molding where resin preforms must be stretched and blown at relatively low temperatures, and where crystallization is to be avoided because it detracts from the appearance and marketability of the polyester resin containers. In the present invention, as described below, it will be noted that in contrast to U.S. Pat. No. 4,123,415, the length to diameter (L/D) ratio of the glass fibers is an important consideration in injection stretch-blowing of polyester resin (PET)-fiber glass compositions and that also the diameter of said fibers and their amount in the compositions are key factors.